Because of their outstandingly good properties with regard to heat- and oxidation-stability, silicon nitride ceramics have great importance as materials in engine construction, e.g. for the use in heat engines, as well as as forming and cutting tools in the case of metal working. The electrical and mechanical properties can thereby be further substantially improved by the introduction of carbide fibers or carbide whiskers (U.S. Pat. No. 4,507,224). Therefore, the production of silicon nitride ceramics strengthened with carbide fibers or carbide whiskers is of great importance.
The product of whisker-strengthened Si.sub.3 N.sub.4 ceramics to give dense materials with improved mechanical properties, as are aimed for especially for engine construction, previously took place only by hot pressing or hot-isostatic pressing of encapsulated powder bodies. However, hot pressing is limited to bodies with simple, uniform geometry.
P. D. Shalek et al., Am. Ceram. Soc. Bull. 65 (1986), 351-265, describe the production of Si.sub.3 N.sub.4 strengthened with SiC whiskers without hot pressing at 1600.degree. to 1850.degree. C., whereby dense bodies were achieved with up to 40 vol. % whisker proportion. However, this hot-isostatic pressing requires an encapsulation and the removal thereof after the consolidation.
Without encapsulation of the powder body, in the case of the process of hot-isostatic pressing, a decomposition of the carbide fibers and whiskers takes place according to the reaction metal carbide+nitrogen.fwdarw.metal nitride+carbon (metal=Si, Hf, Nb, Zr, Ta, Ti, V) which leads to a loss of the property-improving action of the fiber and whisker incorporation.
On the other hand, the high sinter temperature require, however, increased nitrogen pressures in order to prevent a decomposition of the Si.sub.3 N.sub.4 matrix into silicon and nitrogen. Therefore, it is necessary, in the case of pressureless sintering, to use temperatures below 1900.degree. C. in order to avoid a decomposition of the Si.sub.3 N.sub.4 material in the case of the sintering according to the reaction. EQU Si.sub.3 N.sub.4 (solid.fwdarw.3 Si (liquid)+2 N.sub.2 (gas) (1)
In the case of gas pressure sintering (pressures in the range of up to some 10 MPa) and even more in the case of hot-isostatic pressing (up to some 100 MPa) in a nitrogen atmosphere, silicon nitride remains stable up to very high temperatures (e.g. up to above 2700.degree. C. at 100 MPa).
By gas pressure sintering at 1700.degree. to 2000.degree. C. with an N.sub.2 pressure of 10 bar, there could admittedly be produced also bodies with variable geometry with up to 20 wt.-% SiC whiskers but, for a complete consolidation, a proportion of up to 35 mole % sinter adjuvants is then necessary (Tamari et al., YogYo-Kyokai-Shi 94 (1986), 1177-1179). However, this high proportion of sinter adjuvants has the disadvantage that it leads to the formation of a correspondingly high glass portion in the sintered material which manifests itself in a drastic impairment of the mechanical high temperature properties.
However, in the case of hot-isostatic pressing of carbide fiber- or carbide whisker-strengthened silicon nitride, there must also be taken into account the decomposition of the carbide fibers or whiskers according to the above already-given equation "metal carbide+nitrogen.fwdarw.metal nitride+carbon". Therefore, for the total stability of the ceramic body, the decomposition reaction of Si.sub.3 N.sub.4 and the decomposition reaction of the carbide fibers are to be taken into account, where these two reactions stand in a mutual relationship and disadvantageously influence the stability of the other component in question (Si.sub.3 N.sub.4 or carbide fiber).